1. Field of Invention
This invention relates to processes wherein neutron absorbing material is backfilled into semiconductor cavity regions while maintaining low leakage current of the resulting neutron detectors.
2. Background Art and Description of Related Art
The following references are related to the present invention:    Bellinger, S. L., Advanced Microstructured Semiconductor Neutron Detectors: Design, Fabrication, and Performance, Ph.D. Dissertation, Kansas State University, Manhattan, Kans. (2011).    Bellinger, S. L., R. G. Fronk, W. J. McNeil, T. J. Sobering, and D. S. McGregor, “Enhanced Variant Designs and Characteristics of the Microstructured Solid-State Neutron Detector,” Nucl. Instrum. and Meth., A 652 (2011) pp. 387-391.    Bellinger, S. L., R. G. Fronk, W. J. McNeil, T. J. Sobering, D. S. McGregor, “High Efficiency Dual-Integrated Stacked Microstructured Solid-State Neutron Detectors,” IEEE Nuclear Science Symposium, Knoxville, Tenn., Oct. 31-Nov. 5, 2010, in press.    Bellinger, S. L., R. G. Fronk, W. J. McNeil, J. K. Shultis, T. J. Sobering, D. S. McGregor, “Characteristics of the Stacked Microstructured Solid-State Neutron Detector,” Proc. SPIE, 7805 (2010) 7805-0N.    Bellinger, S. L., W. J. McNeil and D. S. McGregor, “Improved Fabrication Technique for Microstructured Solid-State Neutron Detectors,” Proc. MRS, vol. 1164 (2009) L06-01.    Bellinger, S. L., W. J. McNeil, T. C. Unruh, D. S. McGregor, “Characteristics of 3D Micro-Structured Semiconductor High Efficiency Neutron Detectors,” IEEE Trans. Nucl. Sci., NS-56 (2009) pp. 742-746.    Bellinger, S. L., W. J. McNeil, D. S. McGregor, “Variant Designs and Characteristics of Improved Microstructured Solid-State Neutron Detectors,” IEEE Nuclear Science Symposium, Orlando, Fla., Oct. 25-Oct. 31, 2009, pp. 986-989.    Bellinger, S. L., W. J. McNeil, T. C. Unruh, D. S. McGregor, “Angular Response of Perforated Silicon Diode High Efficiency Neutron Detectors,” IEEE Nuclear Science Symposium, Waikiki, Hi., Oct. 28-Nov. 3, 2007.    Bunch, S., J. L., Britton, B. J. Blalock, C. L. Britton, D. S. McGregor, R. Taylor, T. Sobering, D. Huddleston, W. McNeil, T. Unruh, B. B. Rice, S. Bellinger, B. Cooper, L. Crow, “HENDA and Patara: A Solid State Neutron Detector and a Prototype Readout Chip for the SNS”, VIth International Meeting on Front End Electronics for High Energy, Nuclear, Medical and Space Applications, 17-20 May 2006, Perugia, Italy.    Cooper, B. W., S. L. Bellinger, A. Caruso, R. G. Fronk, W. H. Miller, T. M. Oakes, J. K. Shultis, T. J. Sobering, D. S. McGregor, “Neutron Energy Spectrum with Micro-structured Semiconductor Neutron Detectors,” IEEE NSS Conf. Rec., Valencia, Spain, Oct. 23-29, 2011.    De Lurgio, P. M., R. T. Klann, C. L. Fink, D. S. McGregor, P. Thiyagarajan, I. Naday, “A Neutron Detector to Monitor the Intensity of Transmitted Neutrons for Small-Angle Neutron Scattering Instruments,” Nuclear Instruments and Methods, A505 (2003) pp. 46-49.    Dunn, W. L., Q. M. Jahan, D. S. McGregor, W. McNeil, E. L. Patterson, B. Rice, J. K. Shultis, C. J. Solomon, Design and Performance of a Portable Neutron Dosimeter, Proc. 2nd Workshop on European Collaboration for Higher Education in Nuclear Engineering and Radiological Protection, 12-15 Mar., 2006, Valencia, Spain, pp. 85-92.    Gersch, H. K., D. S. McGregor, and P. A. Simpson, “A Study of the Effect of Incremental Gamma-Ray Doses and Incremental Neutron Fluences Upon the Performance of Self-Biased 10B-Coated High-Purity Epitaxial GaAs Thermal Neutron Detectors,” Nuclear Instruments and Methods, A489 (2002) pp. 85-98.    Gersch, H. K., D. S. McGregor, and P. A. Simpson, “A Study of the Effect of Incremental Gamma-Ray Doses and Incremental Neutron Fluences Upon the Performance of Self-Biased 10B-Coated High-Purity Epitaxial GaAs Thermal Neutron Detectors,” Conference Record of the IEEE Nuclear Science Symposium, Lyon, France, Oct. 15-20, 2000.    Henderson, C. M., Q. M. Jahan, W. L. Dunn, J. K. Shultis and D. S. McGregor, “Characterization of Prototype Perforated Semiconductor Neutron Detectors,” Radiation Physics and Chemistry, 79 (2010) pp. 144-150.    Jahan, Q., E. Patterson, B. Rice, W. L. Dunn and D. S. McGregor, “Neutron Dosimeters Employing High-efficiency Perforated Semiconductor Detectors,” Nuc. Instrum. and Meth., B263 (2007) pp. 183-185.    Klann, R. T., Charles L. Fink, Douglas S. McGregor, and Holly K. Gersch, “Development of Coated Gallium-Arsenide for Neutron Detection Applications,” Conference Record of the 12th Biennial RPSD Topical Meeting, Sante Fe, N. Mex., Apr. 14-18, 2002.    Klann, R. T., C. L. Fink, D. S. McGregor, and H. K. Gersch, “Development of Semi-Conductor Detectors for Fast Neutron Radiography,” Conference Record of the 15th Int. Conf. on Applications of Accelerators in Research and Industry, November, 2000.    Klann, R. T., and D. S. McGregor, “Development of Coated GaAs Neutron Detectors,” Conference Record of ICONE-8, 8th International Conference on Nuclear Engineering, Apr. 2-6, 2000, Baltimore, Md. USA.    Lindsay, J. T., C. C. Brannon, D. S. McGregor, and R. W. Olsen, “A Solid State, Position Sensitive GaAs Device as a Neutron Camera,” Fifth World Conference on Neutron Radiography, Berlin, Germany, Jun. 17-20, 1996, pp. 240-248.    McGregor, D. S., S. L. Bellinger, W. J. McNeil, E. L. Patterson, B. B. Rice, J. K. Shultis, C. J. Solomon, “Non-Streaming High-Efficiency Perforated Semiconductor Neutron Detectors and Method of Making the Same,” U.S. Pat. No. 7,855,372; allowed Dec. 21, 2010.    McGregor, D. S., and R. T. Klann, “Semiconductor Gamma Radiation Detector “High-Efficiency Neutron Detectors and Methods of Making the Same,” U.S. Pat. No. 7,164,138; allowed Jan. 16, 2007.    McGregor, D. S., and R. T. Klann, “Pocked Surface Neutron Detector,” U.S. Pat. No. 6,545,281; allowed Apr. 8, 2003.    McGregor, D. S., J. K. Shultis, “Reporting Detection Efficiency for Semiconductor Neutron Detectors; a Need for a Standard,” Nucl. Instrum. and Meth., A632 (2011) pp. 167-174.    McGregor, D. S., W. J. McNeil, S. L. Bellinger, T. C. Unruh, J. K. Shultis, “Microstructured Semiconductor Neutron Detectors,” Nucl. Instrum. and Meth. A608 (2009) pp. 125-131.    McGregor, D. S., S. Bellinger, D. Bruno, W. L. Dunn, W. J. McNeil, E. Patterson, B. B. Rice, J. K. Shultis, T. Unruh, “Perforated Diode Neutron Detector Modules Fabricated from High-Purity Silicon,” Radiation Physics and Chemistry, 78 (2009) pp. 874-881.    McGregor, D. S., S. L. Bellinger, W. J. McNeil, T. C. Unruh, “Micro-Structured High-Efficiency Semiconductor Neutron Detectors,” IEEE Nuclear Science Symposium, Dresden, Germany, Oct. 19-Oct. 25, 2008.    McGregor, D. S., S. L. Bellinger, D. Bruno, S. Cowley, W. L. Dunn, M. Elazegui, W. J. McNeil, H. Oyenan, E. Patterson, J. K. Shultis, G. Singh, C. J. Solomon, A. Kargar, T. Unruh, “Wireless Neutron and Gamma Ray Detector Modules for Dosimetry and Remote Monitoring,” IEEE Nuclear Science Symposium, Waikiki, Hi., Oct. 28-Nov. 3, 2007.    McGregor, D. S., S. L. Bellinger, D. Bruno, W. J. McNeil, E. Patterson, J. K. Shultis, C. J. Solomon, T. Unruh, “Perforated Semiconductor Neutron Detectors for Battery Operated Portable Modules,” Proc. SPIE, 6706 (2007) pp. 0N1-0N12.    McGregor, D. S., S. L. Bellinger, D. Bruno, S. Cowley, M. Elazegui, W. J. McNeil, E. Patterson, B. B. Rice, C. J. Solomon, J. K. Shultis, and T. Unruh, “Perforated Semiconductor Neutron Detector Modules for Detection of Spontaneous Fission Neutrons,” IEEE Conference on Technologies for Homeland Security, Woburn, Mass., May 16-17, 2007.    McGregor, D. S., S. Bellinger, D. Bruno, W. J. McNeil, E. Patterson, B. B. Rice, “Perforated Semiconductor Neutron Detector Modules,” Proc. of 32nd Annual GOMACTech Conf., Lake Buena Vista, Fla., Mar. 19-22, 2007.    McGregor, D. S., et al., “Perforated Semiconductor Diodes for High Efficiency Solid State Neutron Detectors,” presentation recorded in the conference record of the Workshop on Use of Monte Carlo Techniques for Design and Analysis of Radiation Detectors, Coimbra, Portugal, Sep. 15-17, 2006.    McGregor, D. S., M. D. Hammig, H. K. Gersch, Y-H. Yang, and R. T. Klann, “Design Considerations for Thin Film Coated Semiconductor Thermal Neutron Detectors, Part I: Basics Regarding Alpha Particle Emitting Neutron Reactive Films,” Nuclear Instruments and Methods, A500 (2003) pp. 272-308.    McGregor, D. S., R. T. Klann, J. D. Sanders, J. T. Lindsay, K. J. Linden, H. K. Gersch, P. M. De Lurgio, C. L. Fink, and Elsa Ariesanti, “Recent Results From Thin-Film-Coated Semiconductor Neutron Detectors,” Proc. of SPIE, Vol. 4784 (2002) pp. 164-182.    McGregor, D. S., R. T. Klann, H. K. Gersch, E. Ariesanti, J. D. Sanders, and B. VanDerElzen, “New Surface Morphology for Low Stress Thin-Film-Coated Thermal Neutron Detectors,” IEEE Trans. Nuclear Science, NS-49 (2002) pg. 1999-2004.    McGregor, D. S., H. K. Gersch, J. D. Sanders, R. T. Klann, and J. T. Lindsay, “Thin-Film-Coated Detectors for Neutron Detection,” Journal of the Korean Association for Radiation Protection, 26 (2001) pp. 167-175.    McGregor, D. S., R. T. Klann, H. K. Gersch, and Y-H. Yang, “Thin-Film-Coated Bulk GaAs Detectors for Thermal and Fast Neutron Measurements,” Nuclear Instruments and Methods, A466 (2001) pp. 126-141.    McGregor, D. S., H. K. Gersch, J. D. Sanders, and R. T. Klann, “Designs for Thin-Film-Coated Semiconductor Neutron Detectors,” Conference Record of the IEEE Nuclear Science Symposium, San Diego, Calif., Nov. 4-9, 2001.    McGregor, D. S., R. T. Klann, H. K. Gersch, E. Ariesanti, J. D. Sanders, and B. VanDerElzen, “New Surface Morphology for Low Stress Thin-Film-Coated Thermal Neutron Detectors,” Conference Record of the IEEE Nuclear Science Symposium, San Diego, Calif., Nov. 4-9, 2001.    McGregor, D. S., H. K. Gersch, J. D. Sanders, R. T. Klann, and J. T. Lindsay, “Thin-Film-Coated Detectors for Neutron Detection,” Conference Record of the First iTRS International Symposium On Radiation Safety and Detection Technology, Seoul, Korea, Jul. 18-19, 2001.    McGregor, D. S., S. M. Vernon, H. K. Gersch, S. M. Markham, S. J. Wojtczuk and D. K. Wehe, “Self-Biased Boron-10 Coated High Purity Epitaxial GaAs Thermal Neutron Detectors,” IEEE Trans. Nuclear Science, NS-47 (2000) pp. 1364-1370.    McGregor, D. S., J. T. Lindsay, Y-H. Yang, and J. C. Lee, “Bulk GaAs-Based Neutron Detectors for Spent Fuel Analysis,” Conference Record of ICONE-8, 8th International Conference on Nuclear Engineering, Apr. 2-6, 2000, Baltimore, Md. USA.    McGregor, D. S., S. M. Vernon, H. K. Gersch and D. K. Wehe, “Self-Biased Boron-10 Coated High Purity Epitaxial GaAs Thermal Neutron Detectors,” Conference Record of IEEE Nuclear Science Symposium, Seattle, Wash., Oct. 25-29, 1999.    McGregor, D. S., J. T. Lindsay, C. C. Brannon, and R. W. Olsen, “Semi-Insulating Bulk GaAs as a Semiconductor Thermal-Neutron Imaging Device,” Nuclear Instruments and Methods, A380 (1996) pp. 271-275.    McGregor, D. S., J. T. Lindsay, C. C. Brannon, and R. W. Olsen, “Semi-Insulating Bulk GaAs Thermal Neutron Imaging Arrays,” IEEE Trans. Nuclear Science, 43 (1996) pp. 1358-1364.    McGregor, D. S., J. T. Lindsay, C. C. Brannon, and R. W. Olsen, “Semi-Insulating Bulk GaAs Thermal Neutron Imaging Arrays,” Conference Record of the IEEE Nuclear Science Symposium, San Francisco, Calif., Oct. 21-28, 1995, pp. 395-399.    McNeil, W. J., S. L. Bellinger, T. C. Unruh, C. M. Henderson, P. Ugorowski, B. Morris-Lee, R. D. Taylor, D. S. McGregor, “1-D Array of Perforated Diode Neutron Detectors,” Nucl. Instrum. Meth., A604 (2009) pp. 127-129.    McNeil, W. J., S. L. Bellinger, T. C. Unruh, C. M. Henderson, P. B. Ugorowski, W. L. Dunn, R. D. Taylor, B. J. Blalock, C. L. Britton, D. S. McGregor, “1024-Channel Solid State 1-D Pixel Array for Small Angle Neutron Scattering,” IEEE Nuclear Science Symposium, Orlando, Fla., Oct. 25-Oct. 31, 2009, pp. 2008-2011.    McNeil, W. J., S. L. Bellinger, B. J. Blalock, C. L. Britton Jr., J. L. Britton, S. C. Bunch, W. L. Dunn, C. M. Henderson, T. J. Sobering, R. D. Taylor, T. C. Unruh, D. S. McGregor, “Preliminary Tests of a High Efficiency 1-D Silicon Pixel Array for Small Angle Neutron Scattering,” IEEE Nuclear Science Symposium, Waikiki, Hi., Oct. 28-Nov. 3, 2007.    McNeil, W. J., S. Bellinger, T. Unruh, E. Patterson, A. Egley, D. Bruno, M. Elazegui, A. Streit, D. S. McGregor, “Development of Perforated Si Diodes for Neutron Detection,” IEEE Nuclear Science Symposium, San Diego, Calif., Oct. 29-Nov. 3, 2006.    Sanders, J. D., J. T. Lindsay, and D. S. McGregor, “Development of a GaAs-Based Neutron Tomography System for the Assay of Nuclear Fuel,” Conference Record of the IEEE Nuclear Science Symposium, San Diego, Calif., Nov. 4-9, 2001.    Shultis, J. K., and D. S. McGregor, “Design and Performance Considerations for Perforated Semiconductor Thermal-Neutron Detectors,” Nuclear Instruments and Methods, A606 (2009) pp. 608-636.    Shultis, J. K., and D. S. McGregor, “Designs for Micro-Structured Semiconductor Neutron Detectors,” Proc. SPIE, Vol. 7079 (2008) pp. 06:1-06:15.    Shultis, J. K., and D. S. McGregor, “Efficiencies of Coated and Perforated Semiconductor Neutron Detectors,” IEEE Trans. Nuclear Science, NS-53 (2006) pp. 1659-1665.    Shultis, J. K., and D. S. McGregor, “Efficiencies of Coated and Perforated Semiconductor Neutron Detectors,” Conf. Rec. IEEE Nuclear Science Symposium, Rome, Italy, Oct. 18-22, 2004.    Solomon, C. J., J. K. Shultis, D. S. 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Neutron detectors manufactured from semiconductor materials can be classified into two subcategories, those being semiconductor neutron detectors fabricated as rectifying diodes with a neutron reactive coating applied to them, and semiconductor materials that are composed, at least partially, of neutron reactive materials. The former classification is often referred as “coated” or “foil” semiconductor neutron detectors, and the latter classification is often referred to as “solid-form” or “bulk” semiconductor neutron detectors. The present application describes at least one embodiment of an invention within the former classification of semiconductor neutron detectors, in which a neutron reactive material is applied adjacent to a semiconductor rectifying junction.
For the purpose of semiconductor neutron detectors, the most commonly used neutron reactive materials are boron-10 (10B) and lithium-6 (6Li). Although elemental boron can be used, enriched 10B is commonly used to increase the efficiency to maximum. Similarly, it is enriched 6Li that is used rather than elemental Li. Boron and lithium materials used for neutron detection are typically enriched to isotopic concentrations exceeding 95%. The 10B reaction of interest is 10B(n,α)7Li, where 94% of the reactions release a 1.47 MeV alpha particle and a 0.84 MeV 7Li ion in the excited state, thereafter rapidly de-exciting by the release of a 480 keV gamma ray. Typically, the 480 keV gamma ray does not participate in the detection process because it easily escapes the semiconductor substrate and is not absorbed. The remaining 6% of the reactions release a 1.78 MeV alpha particle and a 1.02 MeV 7Li ion. The energetic particles are easily detected, provided that they reach the semiconductor substrate before losing too much energy traveling through the neutron absorbing coating or detector electrical contact. The 6Li reaction of interest is 6Li(n,t)4He, where the reactions release a 2.73 MeV triton and a 2.05 MeV 4He ion. As with 10B, the energetic particles from the 6Li(n,t)4He reaction are easily detected, provided that they reach the semiconductor substrate before losing too much energy in the neutron absorbing coating or detector electrical contact.
A single neutron detector coated with 10B has a maximum theoretical intrinsic thermal neutron (2200 m s−1) detection efficiency of approximately 5%, but is generally measured to be lower than 5% due to electronic noise and reaction product energy attenuation through the electrical contacts. Pure 6Li metal foils applied to a semiconductor diode can yield intrinsic thermal neutron detection efficiencies as high as 12%; however, Li metal is highly reactive and tends to decompose except when treated with stringent encapsulation precautions. For this reason, it is the stable compound 6LiF that is generally used for with coated neutron detectors. Semiconductor diode detectors coated with 6LiF can achieve intrinsic thermal neutron detection efficiencies up to approximately 5.2%, similar to that of 10B-coated devices.
The most common material used for neutron detection is 3He, a rare and expensive isotope of helium gas. These 3He gas-filled detectors are typically pressurized to achieve intrinsic thermal neutron detection efficiencies greater than 75%. However, these 3He gas-filled detectors generally require between 500 volts to 3000 volts to operate, compared to only a few volts needed for semiconductor detectors. Because of their relatively high efficiency, 3He gas-filled detectors remain the preferred choice for neutron radiation detection.
Over the past decade, between 2001 and 2013, a variation on the coated semiconductor neutron detector has allowed for a ten times increase in efficiency over typical coated semiconductor neutron detectors. These new detectors have microscopic structures etched into the substrate. The microstructures then have a pn junction formed on the reticulated surface, or upon the planar surfaces of the semiconductor substrate. Afterwards, 6LiF is backfilled into the cavities of the microstructures. Theoretical calculations indicate that intrinsic thermal neutron detection efficiencies over 35% can be achieved with these detectors, and when two are sandwiched together, thermal neutron detection efficiencies greater than 60% can be achieved. Coupled with the fact that they require less than 5 volts to operate, these detectors have achieved recognition as a promising alternative to 3He gas-filled neutron detectors. Commercialization depends strongly upon a reliable fabrication process that allows for the cavities to be etched such that leakage current is low, and that allows for a non-destructive method to backfill the cavities, while allowing the detectors to be mass produced.
Microstructured semiconductor neutron detectors (MSNDs) are used in neutron detection because of their high detection efficiency and their low power requirements. Trench cavity structures are etched into a semiconductor substrate and then are backfilled with a neutron conversion material that absorbs a neutron and emits charged-particle reaction products. The reaction products enter into the adjacent semiconductor material are subsequently sensed by the semiconductor detector device. The trenches of the devices are high-aspect ratio trenches, generally on the order of 20 micrometers wide by 400 micrometers deep and span the length of the detector diode. Because of this high-aspect ratio requirement of the devices, several difficulties arise regarding the backfilling of neutron reactive materials into the microscopic cavities. An efficient method to backfill these cavities with neutron reactive material is described as part of a preferred embodiment of the present invention.
The first microstructured detectors were fabricated with dry etching techniques, either with capacitive plasma etching systems or inductively coupled plasma etching systems. The inductively coupled plasma etching systems achieve much higher etching rates, as much as 100 microns of etched material per hour, yet even this rate is much too slow for mass production. First, a reaction chamber is limited in capacity, thereby, limiting the throughput of semiconductor substrates per system. Second, the etch rate of 100 microns per hour would require several hours of etching to achieve the required depths needed for microstructured devices backfilled with LiF, typically 400 microns or more. Further, because of the extensive reactor use per etching process, the inductively coupled plasma system must undergo frequent cleaning, thereby, increasing the system down time. It was also learned that the plasma etch process causes damage on the etched surfaces, which manifests as severe leakage current, thereby, reducing the signal to noise ratio of the radiation detectors.
The first silicon-based microstructured semiconductor neutron detectors were fabricated by first making a planar pn diode on the silicon surface. The substrate was often n-type material, upon which a p-type rectifying contact was produced. Afterwards, plasma etched features were cut directly through the pn junction. This process resulted in noisy devices with high leakage current, mainly because of damage caused at the pn junction interface during the plasma etching process.
The second variation of such detectors were manufactured such that the plasma etch did not affect the pn junction, but instead the etched region was recessed away from the pn junction. The leakage current improved significantly, by two orders of magnitude, yet was still too high for practical use.
A third variation of the detectors incorporated the same recessed method as the second method, but an insulating silicon dioxide insulating dielectric was grown inside the etched features in order to remove damaged material and process an electrical insulating layer. This added insulator step improved the devices such that the leakage current was acceptably low, and practical devices could be made. However the leakage current was still higher, by an order of magnitude, than observed for a common Si pn junction diode.
Many methods were used to backfill the etched cavities with LiF material, including physical vapor deposition, ultrasonic vibration, powder compression, flash melting and low pressure condensation. These backfilling methods worked to some amount of success, but had many drawbacks and were time consuming; hence these prior art backfilling methods were not conducive to large scale mass production of the detectors.
6LiF powder is used as the neutron conversion material for most functional MSND devices. Laboratory precipitated and commercially available 6LiF powder is composed of large, micron-sized, cubic crystals as shown in FIG. 1. This large crystal size can cause problems when attempting to backfill the device trenches. Often, the large crystals can block portions of the cavity and prevent it from being completely filled with neutron conversion material, thereby, reducing overall neutron detection efficiency. Examples of inefficient backfilling in the cavities are shown in FIG. 2 and FIG. 3. It is important to reduce individual particle size of the LiF material. Particle size was significantly reduced (see FIG. 4 and FIG. 15) by using a vapor condensation method, as shown in FIG. 16, FIG. 17, and FIG. 18.
Referring to FIG. 1, there is shown LiF crystals formed through a titration process whereupon HF is titrated into a saturated solution of LiOH. The microscopic crystals can be of dimensions generally ranging from submicron to tens of microns. These relatively larger micron range crystal sizes do not pack well into the etched cavity regions of the substrate.
Referring to FIG. 2, there is shown LiF crystals formed through HF titration into LiOH forced into the microscopic etched regions that are 250 microns deep. The result is inefficient packing of material into the substrate, thereby, leaving behind much void space and decreasing the amount of the neutron interactive material per unit volume. The end result is lower neutron detection efficiency than optimum.
Referring to FIG. 3, there is shown a cross section of LiF crystals formed through HF titration into LiOH forced into the microscopic etched regions that are 212 microns deep. The result is inefficient packing of material into the substrate, thereby, leaving behind much void space and decreasing the amount of the neutron interactive material. The end result is lower neutron detection efficiency than optimum.
Referring now to FIG. 4, there is shown LiF nanoparticle material produced through the vapor condensation method. The nanoparticle material can pack efficiently into the etched cavity regions. However, the material can agglomerate into large particles that do not pack well into the cavities.
Referring now to FIG. 5, there is shown fractals of agglomerated nanoparticles of LiF produced through vapor condensation. The nanoparticle material has collected into a series of small spheres through electrostatic forces and then the agglomerated spheres form into fractals of varying dimension.
The following U. S. patent documents are related to the present invention: U.S. Pat. Nos. 4,544,576; 6,545,281; 7,164,138; 7,452,568; 7,855,372 and 2006/0177568.